How to enable ambient occlusion with python?
To turn Ambient Occlusion on you must set its value to True. What you are doing wrong is that you are trying to execute the function use_ambient_occlusion() which does not exist because it's variable not a function.

import bpy
bpy.context.scene.world.light_settings.use_ambient_occlusion = True  # turn AO on
bpy.context.scene.world.light_settings.ao_factor = 0.5  # set it to 0.5